1. Field of the Invention
This invention relates generally to archery bows, and more particularly to compound bows having a single cam.
2. Description of the Related Art
Compound bows comprise a pair of flexible limbs extending from opposite ends of a center handle or riser. The limbs are deflected by the operation of a bowstring in the same fashion as a traditional bow, but the bowstring is connected to the limbs through a system that includes one or more cams and cable stretches that transfer a multiple of the bowstring tension to the limbs. The bowstring and cable stretches may comprise a single continuous loop but, more typically, the bowstring is special bowstring material, and the, cable stretches are aircraft-type cable. The bowstring and cable stretches together are referred to rigging.
Compound bows differ from traditional bows in that compound bows have xe2x80x9clet-offxe2x80x9d leveraging devices typically mounted within a two-tine fork at a free end of each limb, distal from the riser. These leveraging devices are usually pulleys, although they may take various forms, including some with other than circular cross-sections. The leveraging devices are called eccentrics, or cams, because they are pivoted around an axle located off center with respect to their perimeters. In a two-cam compound bow, each limb tip carries a transverse axle upon which a cam is rotatably mounted. In a single-cam compound bow, only one of the limb tips has a camxe2x80x94the other limb tip has a pulley.
Each cam has grooves or tracks similar to the grooves in a conventional pulley. A bowstring is reeved, or wound, around the bowstring pulley and the bowstring cam. A bowstring track is arranged alternately to pay out or take up bowstring as the limbs are alternately flexed to drawn or relaxed to a braced condition. A cam has at least one bowstring track and a cable track. A cable track is arranged alternately to take up portions of the cable as bowstring is paid out while the cam pivots to drawn condition and to pay out portions of the cable as the bowstring is wound onto the bowstring track while the cam pivots to the braced condition.
During operation of the compound bow, the bowstring lengthens as the bowstring is pulled back because as the one or more cams pivot from the braced condition, portions of the bowstring stored in the bowstring tracks unwind and are paid out. Concurrently, portions of the cable are wound onto the cable tracks of the cams so that the cable decreases in length. The opposite occurs as the bowstring is released, permitting the cams to pivot back to their braced condition.
The rigging includes cable stretches oriented approximately parallel the bow bowstring. The cable is commonly positioned to one side of the bowstring to avoid interference with the bowstring. A cable guard rod is mounted to the riser, and a cable guide is slidably mounted on the cable guard rod. The cable guide holds the cable away from the plane of travel of the bowstring.
With a compound bow oriented in its normal position of use, it is conventional to consider the bow as oriented vertically. Therefore, the riser has an xe2x80x9cupper endxe2x80x9d and a xe2x80x9clower endxe2x80x9d. The limb extending from the upper end of the riser may be referred to as an xe2x80x9cupper limbxe2x80x9d terminating in an xe2x80x9cupper limb tipxe2x80x9d. Corresponding terminology is applied to the xe2x80x9clower limbxe2x80x9d that extends from the lower end of the riser.
One problem with prior art compound bows is the presence of unbalanced cable loads. Specifically, the cable loads are unbalanced because the ends of the cables connected to the ends of the limbs are not attached inline with the longitudinal axis of the limbs; during draw, these unbalanced cable loads cause a twisting of the limbs. Frequent longitudinal twisting accelerates fatigue and breakage of limbs. Also, the cable can slip from the grooves of the pulley that is tilted.
A torque or twisting force on the limbs occurs as a result of the cams, which varies as the bowstring is drawn and released. Single-cam compound bows have one circular pulley at one of the limb tips. Limb torque is not a major problem at the limb that carries the pulley because the cable segment can be anchored to the limb at both sides of the pulley, and because the grooves in the pulley can be placed very close to the limb centerline. However, at the limb that carries the cam, limb torque is a major problem because the bowstring and cable segments engage the cam at laterally spaced positions. These cable segments apply a torque through the cam axle to the limb. This problem is exacerbated when a cable guard is employed on the bow because the cable guard offsets the cable segments from the limb centerline. Furthermore, the torque on the limb that carries the single-cam is usually greater than the torque on either limb of a two-cam compound bow, at least in part because of the larger diameter of the cam of a single-cam compound bow.
FIG. 1 shows a prior art single-cam compound bow 100 and its rigging 102. One end of the bowstring 104 is at least partially wound around a first bowstring track in the cam 106 at the bottom limb tip 108. The bowstring 104 extends across the bow in a first stretch 105, and is wound around a bowstring pulley 110 at the top limb tip 112. The bowstring 104 extends across the bow again in a second stretch 113 and the other end of the bowstring is at least partially wound around a second bowstring track in the cam 106 at the bottom limb tip 108. A cable 116 is at least partially wound around a cable track in the cam 106, extends across the bow in a single stretch 117, and terminates with a split harness 118 at the top limb tip 112.
Torque is applied to cam 106 about axes in two separate planes. Rotational torque is applied about the axis of rotation of the cam. Limb torque is applied about an axis in a plane normal to the axis of rotation of the cam. Limb torque attempts to tip the cam sideways and is opposed by the limbs, thus developing a torsion in the limb tips.
Rotational torque is applied to the cam by the force of the bowstring acting in the bowstring groove of the cam and by the force of the cable acting in the cable groove. The lever arm through which each of the cable and bowstring applies its torque is the distance from the axle to the point of tangency of the cable or bowstring in the groove.
When the pulley is not rotating, such as when the bow is held at full draw, the rotational torque applied by the cable and by the bowstring are equal and in opposite directions. The force applied by the bowstring multiplied by the lever arm in the bowstring groove, equals the force applied by the cable multiplied by the lever arm in the take-up groove.
Total limb torque applied to the limbs is determined by the magnitude of the force vectors applied by the bowstring and cable sections and by their respective placement along the pulley axle. The axis about which limb torques are applied, the limb torsional axis passes through and extends normal to the rotational axis of the cam. The exact location of the limb torsional axis depends upon the composition and structure of the limb. Therefore, the distance along the axle upon which the cam rotates defines the lever arm for that force, and the limb torque produced by the force equals the magnitude of the force multiplied by the lever arm for the force.
With prior art single-cam bows, limb torque causes cam axle bushings to be replaced annually, whereas without excessive limb torque, the bushings would last the life of the bow.
With prior art single-cam compound bows, limb torque causes undue bowstring wear and bowstrings last only six months because they have a tendency to come out of the cam groove.
Prior art single-cam compound bows suffer from vibration, noise and frictionally introduced hesitation effects as the cable guide moves along the cable guard rod.
Prior art single-cam compound bows lack a split harness at the free end of the limb having the cam.
It is therefore the object of the present invention to provide a single-cam compound bow in which torque applied to the power-cam limb, as the bow is drawn and released, is reduced or eliminated.
It is another object of the present invention to provide a compound bow having a reduced failure rate.
It is still another object of the present invention to provide a compound bow having reduced vibration.
It is yet another object of the present invention to provide a compound bow having a longer lasting bowstring.
It is yet another object of the present invention to provide a compound bow having reduced wear on axles of idler pulleys.
It is a further object of the present invention to provide a torqueless buss cable positioner for compound bows, whereby the buss cables are laterally displaced with respect to the bow plane, while involving very little bow noise and vibration.
It is a further object of the present invention to provide a shorter cable guard rod.
It is a further object of the present invention to provide a compound bow in which the distance that the cable guide travels on the cable guard rod is minimized.
It is a further object of the present invention to provide a bow with counter-rotating idler pulleys.
It is still a further object of the present invention to provide a compound bow having a lighter spine limb.
These and other objects of the present invention will become apparent to those skilled in the art as the description thereof proceeds.
Briefly described, and in accordance with a preferred embodiment thereof, the present invention relates to a single-cam compound bow that incorporates a riser, a first limb and a second limb. Each limb has an inner end connected to the riser and a free outer end. Two stretches of a bowstring extend between the free outer ends of the limbs, and two stretches of cable extend between the free outer ends of the limbs.
Other aspects, features and advantages of the present invention will become apparent to those skilled in the art from the following detailed description and the accompanying drawings. It should be understood however that the detailed description and specific examples, while indicating preferred embodiments of the present invention, are given by way of illustration only and various modifications may naturally be performed without deviating from the present invention.